of comparing angles created by the lines and the horizontal axis. If the marginal angle is greater than the average angle then the function is elastic at the point; if the marginal angle is less than the average angle then the function is inelastic at that point. If, however, one follows the convention adopted by economists and plots the independent variable P on the vertical axis and the dependent variable Q on the horizontal axis, then the opposite rules would apply. The same graphical procedure can also be applied to a supply function or other functions. Semi-elasticity A semi-elasticity (or semielasticity) gives the percentage change in f(x) in terms of a change (not percentage-wise) in x. Algebraically, the semi-elasticity S of a function f at point x is The semi-elasticity will be constant for exponential functions of the form, since, An example of semi-elasticity is modified duration in bond trading. The term "semi-elasticity" is also sometimes used for the change if f(x) in terms of a percentage change in x which would be See also Arc elasticity Elasticity (economics) Elasticity coefficient (biochemistry) Homogeneous function Logarithmic derivative References Further reading Functions and mappings Mathematical economics